This invention relates generally to spectrographs, and, in particular, to an improved optical configuration that includes means for passive thermal compensation.
In optical systems that include multiple lenses or other elements, temperature changes may affect the relative positions of the elements due to thermal expansion or contraction. Other material properties, such as the refractive index of the elements, may also change with temperature.
In precision optical systems, these changes in position and properties can cause shifts of focal length, wavelength, or other parameters, reducing the accuracy of measurements made with such systems. As an example, the distance between a lens and a fixed image plane (such as a detector array) will typically increase with temperature due to expansion of mechanical parts holding the lens and the image plane. The focal length of the lens, however, typically does not change with temperature to the same extent as the change in distance. As a consequence, the image will not be in sharp focus but will be blurred at the image plane.
This problem has been addressed by a variety of techniques involving passive or active thermal compensation. For example, U.S. Pat. No. 4,190,325 to Moreno discloses a cylindrical configuration in which compensatory lens movement is provided by an arrangement of push rods and levers. This configuration is relatively complex, however, and would be expensive to manufacture.
U.S. Pat. No. 4,236,790 to Smith discloses an alternative cylindrical configuration in which concentric cylinders are separated by a material having a relatively large thermal coefficient of expansion. The separation material expands with temperature so that it moves the inner cylinder in a direction and by an amount to compensate for thermal expansion of the housing of an optical system. The configuration is relatively complicated, requiring several concentric cylinders and provision for accurate guiding of linear relative motion between two of the cylinders.
In U.S. Pat. No. 4,525,745, Ghaem-Maghami et al disclose the use of bimetal rings to move a lens element in a manner to compensate for thermal changes. The use of a bimetal strip is also shown by Ghaem-Maghami et al, but the strip is shown constrained at both ends, which would negate the advantages of its being bimetallic. Bimetallic strips are typically constrained only at one end so that the other end is free to move when acted on by temperature changes.
U.S. Pat. No. 4,861,137 to Nagata discloses a mounting configuration that uses annular bimetallic elements to move a resin lens along its optical axis to compensate for thermally induced changes in the separation between a laser diode and its collimating lens. This approach requires a complicated mounting system and is limited to small lenses.
U.S. Pat. No. 5,557,474 to McCrary discloses still another approach for passive thermal compensation that uses materials with different thermal coefficients of expansion and angled interfaces to control movement as a function of temperature in a way that maintains fixed separation distances between optical elements. The McCrary patent further discloses the use of nested arrangements of the approach so as to control distances between elements within subgroups of optical elements while simultaneously controlling distances between the subgroups. Although this patent discloses a method for providing lateral movement in response to temperature changes, it appears susceptible to introducing tilt as well as pure lateral displacement.
Existing active thermal compensation techniques are typically more complex than passive techniques because of the requirement for sensors, electronic signal processors, and the active application of mechanical force. Such systems tend to be bulky, heavy, and expensive compared to passive thermal compensation designs.
In certain optical systems, it is necessary to control movements that are in a direction lateral to the optical axis. An instrument such as a spectrograph, for example, introduces folds into the optical path and as a result lateral movements of its optical elements caused by temperature changes may degrade its performance. FIG. 1 shows the configuration of a spectrograph in which the various optical components are mounted to a common base. Temperature changes cause the base plate to expand or contract, respectively increasing or decreasing the separation of the optical components in two dimensions. In addition to causing movements along the optical axis that defocus the spectra at the detector array, temperature changes also cause lateral movements that can shift the location on the detector array of a given spectral line. For example in a representative prior art spectrograph, temperature changes move spectral peaks across the face of the detector array by about 1 xcexcm/xc2x0 C. Such lateral movements reduce accuracy of spectral measurements made with the instrument. Prior art methods of compensating for the spectral shift required frequent calibration measurements that were time consuming and required the addition of calibration equipment.
There is a need, therefore, for a method of thermal compensation that provides both longitudinal and lateral movements of optical components to offset the effects of temperature changes in optical instruments. It is desirable that the method be simple and inexpensive to avoid adding unnecessary weight and cost to the instrument. Preferably the method should be one of passive thermal compensation.
This invention is directed to reliable and economical passive thermal compensation for optical apparatus, including spectrographs. The invention provides for such compensation in both focus (parallel to the optical axis) and lateral position (perpendicular to the optical axis).
Different embodiments ensure that a lens maintains a stable position relative to a detector array in the presence of temperature fluctuations. To maintain the position of the lens along the optical axis, two or more polymer spacers are used between the lens mount and a floating flange to which the lens is attached. The polymer spacers have a thermal coefficient of expansion such that when the temperature increases the lens is moved toward the detector array by the spacers to compensate for the normal increase of the lens-detector spacing with temperature.
As a separate aspect of this invention, which may or may not be used in conjunction with the polymer spacers, flexure mounts are used to connect the lens mount to the floating plate, preferably one on each side of the lens. Each flexure mount is part of a bimetallic strip that bends by a predetermined amount in a known direction when the temperature changes, thereby moving the lens in a direction lateral to the optical axis. In the case of a spectrograph, this lateral motion maintains the positional stability of given spectral lines on their respective detector pixel elements.